A high silver loading in an ethylene oxide catalyst is generally known to result in an increased selectivity, activity, and stability. For these reasons, a high silver loading is generally desirable. One method known in the art to increase the silver loading in a catalyst is the use of a high water absorption carrier. However, use of a high water absorption carrier generally leads to the loss of mechanical strength in the catalyst. Another method known in the art to increase the silver loading in a catalyst is the use of a double impregnation process. However, double impregnation generally lowers catalyst productivity and adds complexity to the process when incorporating promoting species.
Conventional silver impregnating solutions of the art typically contain silver oxalate, or a combination of silver oxide and oxalic acid, together in ethylenediamine (EDA). In the conventional oxalic acid formulation, oxalate in oxalic acid provides oxalate anion (C2O42−) to counter the charge balance of silver cation (Ag+) in the EDA/water solution to form a soluble [Ag2C2O4]-EDA complex. For this reason, the oxalic acid is often referred to as an “anion contributor”. The maximum permissible silver concentration in these solutions is generally 31 wt % under ambient conditions. Attempts to increase the silver concentration beyond this generally result in precipitation of silver salts, and thus, the silver concentration in conventional silver solutions is generally adjusted to no more than 31 wt %.
As indicated, there would be a significant benefit and advantage in an impregnating solution that could retain a silver salt in significantly higher concentrations than permitted in the art, while being stable during storage and use, i.e., with complete dissolution of silver in the absence of precipitation. Such an impregnating solution would be useful in producing ethylene oxide catalysts having a high silver loading by more straight-forward means, without the complexity and other drawbacks of conventional methods of the art, such as use of a high water absorption carrier or a double impregnation process.